Printing processes include stencil duplicating (mimeography) and screen-printing. These are processes which utilize stencils and in which ink is allowed to pass through ink permeable areas on the stencil (image areas) to the paper, thus printing an Image. Duplicating, usually a rotary process, is used for printing finer details than screen printing which is a flat bed process.
Various stencil making processes have been developed and used over the years, both for mimeography and screen-printing. The process of stencil making involves cutting out image areas in the stencil. This was originally done by hand cutting of the stencil material, Other methods used various mechanical means to cut the image areas in impregnated tissue or in free standing films. Electro-optical methods were also used for stencil cutting by IR absorption or Electro-erosion. Since the 1980's, thermal digital duplicators have been developed. In these machines, the stencil is cut using a thermal head to make holes in a thin polyester layer.
Examples of existing printing stencils and of their preparation are illustrated in FIGS. 1 and 2.
FIG. 1 illustrates a thermal stencil and describes the method for its preparation. The thermal stencil 10 consists of a thin polyester film 12 laminated on a fine tissue base 14. Holes 16 are made in the polyester film 12 using a thermal head 18. During printing, ink 15 becomes accessible to paper 13 through holes 16. This technology gives reasonable quality, short preparation time and is suitable for a digital work flow. It is used in all digital duplicators made at present.
FIG. 2 illustrates a laser thermal stencil and describes the method for its preparation. The laser thermal stencil 20 consists of an IR sensitive layer 22 and a thin polyester film 24 laminated on a paper tissue base 26. IL patent U.S. application No. 09/402,205, assigned to the common assignees and co-inventors of the present invention, discloses the use of a laser beam for cutting a stencil similar in its structure to the thermal stencil. Instead of using an electric thermal element, a laser beam 28 is used to generate the heat in a radiation-absorbing layer, such as the IR sensitive layer 22, cutting holes 21 in the IR sensitive layer 22 and polyester layer 24. During printing, ink 25 becomes accessible to paper 25 through holes 21. A high-resolution stencil may be quickly produced in this method. So far there is no commercial implementation of this method.
An example of an existing UV stencil for screen printing and of its preparation is illustrated in FIG. 3.
Stencil 30 is made of a UV curable material (photo polymer) 32 and a fine mesh 38. A photographic film 34, containing image 33, is laid over the photo polymer 32, and acts as a mask during the exposure of stencil 30, in a contact exposure process. The UV curable material hardens as it is exposed to UV light 36 through transparent areas 31 in the photographic film 34. The film 34 is removed and the stencil is washed out, in the developing process, leaving the mesh 38 and the cured material 39. The stencil is then used for printing ink 37 on paper 35. Ink 37 is inaccessible to paper 35 in the areas defined by the cured material 39, and will contact paper 35 only in the non cured areas 33' around the cured material 39. The Screen-printing stencils thus produced have relatively high quality. Numerous versions of this principle exist, with direct (on-mesh processing) and indirect (off-mesh processing) emulsions and photo polymer films.
FIG. 4 illustrates a digital flexographic printing plate and describes the method for its preparation, such as described in U.S. Pat. Nos. 5,262,275 and 5,607,814. The flexographic printing process is a relief printing process which does not utilize stencils. Flexographic plate 40 consists of a UV sensitive photo polymer 48 coated by a dense black IR sensitive layer 42. Layer 42 is etched out by a powerful IR laser 44, forming an integral mask having holes 43 through which the plate 48 is exposed to UV light 46. The UV exposure from light 46 forms, by curing, a relief image 41 on the plate. Additional exposure to UV lamp 49 forms a cured plate base 41'. After processing, relief image 41 can be used for printing ink 47 on paper 45. The process requires powerful exposures for both IR and UV light. It is as lengthy as for normal UV plates but it gives high quality since contact exposure with a film is no longer required.
UV curable material was utilized for screen printing, as described in FIG. 3 and for flexographic plate printing, as described in FIG. 4, but has never been utilized in the preparation of printing duplicating stencils and has never been used in an on-press process
The existing methods for producing printing stencils for duplicating (such as described in the above patents and in FIGS. 1 and 2) are limited in the quality of stencils which can be produced due to various effects of the thermal interaction of the material either with the thermal head or with the laser. Present stencil duplicating is limited in its application to simple jobs with very coarse halftone images.